The present invention relates generally to EMI shielding. More specifically, the present invention relates to EMI shielding that comprises an absorbing material that is capable of absorbing electromagnetic radiation.
All electronic products emit electromagnetic radiation, generally in the range of 50 MHz. to 1 GHz., but not limited to this range especially in light of the many advances in high-speed microprocessor design and the rapidly increasing capabilities of high-speed networking and switching. The problem of emittance of electromagnetic radiation is not new to designers of electronic equipment; indeed, significant efforts are taken to reduce electromagnetic interference (EMI) and electromagnetic radiation (EMR) and virtually every country has a regulating agency (FCC in the U.S., for instance) that controls the marketing and sale of electronic equipment that do not pass stringent requirements for EMI/EMR, whether radiation or intercepted (also called susceptibility) by an electronic device.
Present day solutions typically involve the use of conductively painted plastic housings, conductive gaskets, and/or metal cans that are affixed to a printed circuit board by soldering or similar methods, some of which are semi-permanent. In virtually all cases, the existing solutions are expensive and add to the cost of manufacturing electronic equipment such as cell phones, personal digital assistants, laptop computers, set-top boxes, cable modems, networking equipment including switches, bridges, and cross-connects.
More recently, technology for the metallization of polymer substrates has been in evidence. For example, Koskenmaki (U.S. Pat. No. 5,028,490) provides a polymer substrate that is layered with aluminum fibers and sintered to form a flat material with a metal layer that is intended to provide EMI control (also a legal requirement of the FCC and other foreign entities and generally referred to as electromagnetic compliance or EMC). This product was manufactured and sold by the 3-M Corporation. As of approximately 2002, this product was withdrawn from the market. The material was shown to be expensive, difficult to use, and subject to inferior performance due to cracking of the metallized layer.
Gabower (U.S. Pat. No. 5,811,050) has provided an alternative approach wherein a thermoformable substrate (any number of different kinds of polymers) is first formed, and then metallized. This approach offers the advantage of not subjecting the metallized layer to the stresses created during molding. The product has been shown to be highly effective and relatively low-cost.
The major methods of providing for a conductive coating or layer on a substrate generally include (1) selective electroless copper/nickel plating, (2) electroless plating, (3) conductive paints and inks, and (4) vacuum metallization. Collectively, these are referred to herein as “metallization methods.” In each of these typical applications, either a planar or formed substrate of metal or plastic is “treated” to form a conductive shield. The ultimate quality, performance, and cost for each method varies widely but ultimate a metallized thermoformable shield is formed into an (1) integral solution that surrounds the printed circuit board in some manner (a.k.a., “enclosure” level solution), or (2) formed into a compartmentalized shield that fits on the guard traces of the PCB (“board” level solution), or (3) formed into smaller shields that fit over individual components using the guard traces (“component” level solution).
When it comes to EMI shielding at the board of component level, the feature deployed today is to place a conductive surface of the EMI shielding in contact with the guard traces either (1) directly by metallizing a shield surface and placing it in contact with the trace or (2) by metallizing the “outside” surface (from the perspective of the component being shielded) and then using some method of attachment that connects the guard trace with the metallized outside surface. The purpose of the guard traces, based upon the historical use of soldered metal cans, is to provide a point of contact between the EMI shielding and the PCB that can be subject to standardized surface mount technology (SMT) solder reflow processes that ultimately provide a solid connection between the metal can shield and the PCB. And, while the metal can and guard trace become grounded at least one point to the signal, power, or ground plane(s), the amount of peripheral contact between the shield and metal can is largely for the purpose of achieving a tight mechanical seam.
The resultant assembly of the EMI shielding and the electronic component provides adequate shielding for many applications; however, as the frequency of chips increase and the data transmission rates increase, the creation of errant radiation (EMI) becomes much easier and more harmful to circuits and components located nearby. Indeed, with the increasing density of chips, the subject of immunity (of one chip relative to another) becomes all the more important. Thus, in general, conventional EMI solutions will increasingly find themselves inadequate for purposes of immunity and indeed, radiated emissions may also become an increasing issue. Furthermore, for microwave devices, especially those that operate of have harmonic frequencies above about 10 GHz. radiated emissions will be a significant concern.
The radiation fields within an inner space defined by a printed circuit board and an EMI shield are comprised of very complex combinations of both electric fields (E-fields) and magnetic fields (H-fields) that are bouncing off chip and shield structures forming very complex fields with many resonances. These resonances can be very strong in terms of field strength and can easily be observed at frequencies that are troublesome from an EMC perspective. In general, there is nothing to contain the radiation escaping from the bottom of the chip except for the phenomena of reflection from the ground plane (the “image” plane) which can, in some situation, improve the radiation emissions problem but is problematical from a design and manufacturing point to achieve.
While electromagnetic radiation (EMR) fields are very complex, the behavior of EMI shielding can be determined from a measurement of shielding effectiveness (SE). Typically, this is done in the far field where the EMR fields are distinctly plane wave in form. In the near field, EMR is either reflected or absorbed and for the most part, with the drive for lightweight devices and shielding, reflection has been the only viable method of shielding. Controlling EMR is then a matter of designing either solid or intermittent shielding around the chip that increases the SE.
A phenomenon that may limit the effectiveness of EMI shielding structures is often referred to as cavity or enclosure resonances—the tendency of enclosed spaces with reflective surfaces to develop standing waves of varying amplitudes. It is not unusual for the peaks of the standing waves to exceed limits for radiated emissions. The wavelength of these resonances in a simplified sense is a function of a half wavelength, the dimensions of the structure and its configuration (square, rectangle, circle, etc.) with a constraint involving whole numbers of half wavelengths. In real situations, these cavity resonances are detuned (shifted in frequency and reduced in amplitude) from theory by the presence of intermediate or internal structures (i.e., chips and electronic components). In general, only sophisticated numerical analysis codes can predict these resonances. More importantly, there are not any readily acceptable methods to altering the frequency of the resonances or their amplitudes.
The presence of these difficult-to-treat resonances makes EMC design difficult. Therefore, what are needed are methods and EMI shielding that can absorb resonances within the EMI shielding.